Polyoxometalates (POMs) are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge, which is balanced by cations that are external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group IIIa-VIa elements such as phosphorus, antimony, silicon and boron. The framework of polyoxometalates comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Due to appropriate cation radius and good π-electron acceptor properties, the framework metal is substantially limited to a few elements including tungsten, molybdenum, vanadium, niobium and tantalum.
In the past, there have been increasing efforts towards the modification of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility etc.
For example, Contant et al. report on the crown heteropolyanion [H7P8W48O184]33− (R. Contant and A. Tézé, Inorg. Chem. 1985, 24, 4610-4614; R. Contant, Inorg. Synth., 1990, 27, 110-111). This polyanion is composed of four [H2P2W12O48]12− fragments which are linked by capping tungsten atoms resulting in a cyclic arrangement having a large central cavity. [H7P8W48O184]33− is described to be rather stable in aqueous solution and to yield no complexes with di- or trivalent transition metal ions.
Nevertheless, Kortz et al. disclose the use of [H7P8W48O184]33− as a superlacunary polyanion (Angew. Chem. Int. Ed. 2005, 44, 3777-3780). The interaction of CuCl2 with K28Li5[H7P8W48O184] in aqueous medium at pH 6 results in the formation of the large wheel-shaped anion [Cu20(OH)24(H2O)12(P8W48O184)]25−. During this synthesis the structure of the annular [H7P8W48O184]33− precursor is maintained and its cavity is filled with a highly symmetrical copper-hydroxo cluster bearing terminal water ligands. [Cu20(OH)24(H2O)12(P8W48O184)]25− and its qualities such as redox and electrocatalytic properties have been the target of several studies (Nadjo et al., Electrochemistry Communications 2005, 7, 841-847; Kortz et al. Inorg. Chem. 2006, 45, 2866-2872; Kortz et al. J. Am. Chem. Soc. 2006, 128, 10103-10110).
Moreover, there have been remarkable efforts to prepare and study other (P8W48O184)-based polyanion structures using other transition metals. However, up to now only the synthesis of lanthanide-containing {Ln4(H2O)28[KP8W48O184(H4W4O12)2Ln2(H2O)10]13−}x, Ln=La, Ce, Pr, Nd is reported (Kortz et al., Inorg. Chem., 46 (5), 2007, 1737-1740, web release date: Feb. 13, 2007, DOI 10.1021/ic0624423). The central cavity of this polyanion is occupied by two W4O12 groups, two potassium ions and four lanthanide cations which have an occupancy of 50%.
It is the object of the present invention to provide further transition metal substituted (P8W48O184)-based polyoxometalates. Such transition metal substituted POMs should be useful as catalysts in homogeneous and heterogeneous oxidation reactions of organic substrates. In addition, they should be easy and reproducible to prepare.